Histoplasma capsulatum is a facultative intracellular parasite of mononuclear phagocytic cells. Peritoneal macrophages from mice immunized by sublethal infection restrict the intracellular growth of H. capsulatum. Splenocytes from immune mice arm normal macrophages to suppress intracellular growth of the fungus. This activation of normal macrophages is mediated by lymphokines found in supernatants from cultures of antigen-stimulated immune splenocytes or those from concanavalin-A (ConA) stimulated T cell hybridomas. The lymphokines have no direct effect on H. capsulatum but act by stimulating macrophages to suppress fungal growth. Current evidence indicates that gamma interferon (IFN-Gamma) is the lymphokine that stimulates macrophages to inhibit H. capsulatum. Macrophage suspensions are comprised of subpopulations some of which are responsive and others of which are unresponsive to lymphokine stimulation. Responsive cells synthesize a set of unique peptides when stimulated by lymphokines, and prevention of protein synthesis in macrophages antagonizes their abiliity to inhibit intracellular growth of H. capsulatum. The use of a macrophage cell line in place of the more heterogeneous native peritoneal populations would appear to offer advantages in further studies on mechanisms of intracellular inhibition. The Specific Aim of the work described in this proposal is to identify molecular bases for the inhibition of intracellular growth of H. capsulatum within macrophages activated by IFN-Gamma. The aim will be pursued by analyzing the antihistoplasma state induced by recombinant IFN-Gamma in resident peritoneal macrophages of C57/B16 mice and in a responsive macrophage cell line, P388D1. The analysis will be conducted by: (i) measuring the intracellular growth of H. capsulatum within the cells, (ii) recording the occurrence and characteristic unique peptides formed by cells stimulated by IFN-Gamma, and (iii) assessing the potential role of the peptides in the antihistoplasma state of the macrophages.